Man and woman are in a committed relationship or seven years and have a child together. She has no sex drive. Nothing is wrong with her, she just doesn't want to. Does that justify him getting it elsewhere? Yes or no, and why?
Edit:
There have been issues with depression in the past. And when it was, it was treated with counseling and therapy as well as a variety of different medications. The meds worked but made the desire for sex even less which increased the problems and fighting. The meds caused bigger issues for her such as excessive sweating, insomnia, and lack of a back bone meaning that she went with the flow of life with no emotions and everyday was a blur.
So she quit with the meds and he threatened to leave her if she didn't get back on them. She couldn't win for losing. There are other issues involved as well. He is a hard working man and a good provider. However, he provides no help at home with child, chores, regular maintenance of owning a house, etc. she works too. She stays frustrated at his lack of interest in his role at home and constant concern of just sex. He doesn't take care of his hygiene like he did when the relationship first started such as brushing teeth regularly (that's a big thing). Even with this, she IS attracted to him but doesn't know how to approach it. Also, there are lots of underlying issues with his family. On multiple occasions he has allowed his family to treat her poorly and has refused to stand up to them on her or their child's behalf, right or wrong. And with time the relationship has turned into simply coexisting for their child's sake. Her heart is still there though, or she would have left for good instead of always coming back. He doesn't see where he does wrong, he just feels like he is being done wrong. She has no desire to be with him or any man for that matter. He isn't concerned with fixing any problem but the lack of sex. He had not stepped out that she is aware. With him working like he does, he doesn't have time. And he is home when he is suppose to be. He doesn't go out.

Bump! You want more details. Her sex drive has been lacking, to say the least, for the last 5 years. She has gone to the doctor and there are no physical issues. Depression, yes. And she couldn't win for losing there because the meds made the problem worse. Sex is like a chore. It causes lots of tension in the relationship. He is very frustrated by this. Frustrated to the point of stepping out? I'm not sure. But would it be justified if he did? She knows its a problem and doesn't care to fix it. Thoughts?

are they married?...makes a difference IMO. If so, no he has no right. If not, he has the right to be in another committed relationship...but not cheat.

has she tried hormone therapy?

is she doing what she can to want it? or just does not care?

ETA: just saw answer to the last couple of questions. if she really doesn't care it seems like things are over. sex is a big part of a relationship. If she isn't at least willing to make things better I don't see why he should stay. for the record, I am not condoning cheating. I absolutely never think that is ok, ever.

No, they need to go to counseling so he can communicate how important the sexual relationship is to him and she can figure out why she has no desire. If he decides that having a sexual connection is something that he can't do without and his wife is unwilling to work on her lack of desire then he needs to end the marriage.

No ... never justifies a reason to go outside the relationship ... NEVER. Going outside the realationship just adds to the problem. Unfortunationly, the fact that anonymous doesn't care to fix her lack of any desire and/or the problem, it seems to me she isn't giving her SO much incentive. They really need to get some councelling and she really needs some councelling and medical help on her own. A sexual relationship is a very important part of any relationship, even though it's not the end all of a relationship.